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What is a quantum coin toss? - YouTube
Channel: Physics Girl
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[Music]
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hey y'all i'm diana you're watching
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physics girl and i want to tell you
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about a well-known problem in the field
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of quantum cryptography the problem is
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basically flipping a coin over the phone
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but let's see why that might cause
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problems
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hey jade hey are we still on for the
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beyonce concert friday night that's the
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same night as the star site concert oh
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well what if we caught the coin if it's
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heads we go to beyonce and if it's tails
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we go to star sets okay i'll flip it
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okay but let's facetime so i know you're
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not cheating oh my phone's gonna die i
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can't okay fine
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looks like it landed on tails star said
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here we come oh it totally did not land
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on tails you just say you always do this
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we always just end up doing what you you
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see
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but we could do a quantum coin flip
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which is the obvious solution for a
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situation like this between two parties
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who don't trust each other a quantum
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coin flip goes like this first we need
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to know a basic principle of quantum
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physics known as wave function collapse
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which states that measuring a quantum
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object like a tiny particle changes it
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for example photons the particles of
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light have a quantum property called
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polarization polarization is like
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light's orientation
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because yes light travels in a direction
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but it also waves in a direction like it
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can wave up and down but it can also
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wave side to side and diagonally and the
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direction it waves is the polarization
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guess what direction normal light like
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light from sunlight or from a lamp is
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polarized trick question it's
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unpolarized because it has waves of all
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different and random polarizations mixed
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together but we can make light wave in
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one particular direction by passing it
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through a filter like pass it through a
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vertical filter and it'll come out
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waving up and down but if we try to pass
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light that's already vertically
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polarized through a horizontal filter
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it'll be blocked photons can't pass
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through filters that are at 90 degree
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angles to their polarization however and
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this is the interesting part if we try
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and pass a photon through a filter with
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a 45 degree of difference like vertical
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to diagonal something weird happens half
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of the time it goes through and half the
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time it gets blocked and there's no way
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to predict which it'll do it's
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completely random and the photons that
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make it through are now diagonally
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polarized which means the act of
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measuring the light's polarization
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changes it that
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is wave function collapse and it can be
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demonstrated
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using polarizing filters
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in a demo that has blown many a mind
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including mine check it out vertical
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polarizing filter some light goes
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through horizontal filter all the light
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is blocked but when i add a diagonal
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filter in between some light can make it
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through this is the weird thing about
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quantum objects they act based on
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probabilities so when the vertically
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polarized photons come to the diagonal
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filter they act completely based on
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chance so half of them make it through
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and half of them don't the ones that
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make it through are now diagonally
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polarized and so now when those get to
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the horizontal filter they're again at a
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45 degree angle so now again
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half of them make it through so cool now
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we can use this wacky property of
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photons to make sure no one cheats this
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is what's going to happen diana's going
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to choose one type of orientation
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vertical and horizontal which is called
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rectilinear or diagonal then using only
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that orientation she's going to send me
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a stream of photons which will encode a
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random sequence of ones and zeros i'm
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going to record these ones and zeros and
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try and guess which orientation she
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chose
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if i'm right i win
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if i'm wrong she wins
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here's an example okay
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say i choose the rectilinear measurement
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orientation don't tell jade so i only
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use the vertical and horizontal filters
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on my photons now i make a random
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sequence of ones and zeros on my
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computer and then i encode or match
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these ones and zeros into photons by
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saying a vertically polarized photon
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means a one and a horizontally polarized
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photon means a zero and finally i'll
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send my photons off to jade via a secure
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channel jade i'm sending you some
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photons okay i've polarized them so that
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these the vertical and first diagonal
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are ones and these the horizontal and
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the other diagonal are zeros wait how
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will i know which filter to use you
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won't
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jade doesn't know which orientation i
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chose so she alternates randomly between
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using a rectilinear and a diagonal
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filter when she measures a vertical
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photon with a vertical filter it passes
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through and she notes down a one when
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it's a horizontal photon and it gets
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blocked she writes down a zero but when
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she uses a diagonal filter to measure a
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vertical photon there's a 50 50 chance
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it gets blocked and when that happens
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she has no way of knowing if it was
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vertical horizontal or just the wrong
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type of diagonal but because it gets
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blocked she notes down a zero the same
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thing could happen with a horizontal
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photon it could go through and in that
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case she would wrongly note down a one
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okay i've recorded them all and noted
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them down in separate tables okay guess
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whether i've polarized them using a
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rectilinear or a diagonal filter her
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recordings don't give away any
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information as to which orientation i
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chose so just like heads or tails she
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has a 50 50 chance of getting it right
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uh
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diagonal
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no it was rectilinear oh you totally
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just cheated again how am i supposed to
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believe you now to prove that i'm not
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cheating i just read out my sequence of
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ones and zeros it should match exactly
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what jade got in her rectilinear side of
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the table if we imagine for a second
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that she correctly guessed rectilinear
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and i tried to cheat by saying it was
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diagonal which of course i would never
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do i would have to guess all the ones
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and zeros in the diagonal column and i'd
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have to get those correct but if we used
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enough photons that'd be pretty much
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impossible okay here it goes one zero
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one zero zero zero one one one one zero
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one it's rigged i'm staying home friday
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night
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as jade knows because she's also a
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physicist there's no way to rig this
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system it was just an unfortunate
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outcome thank you so much for watching
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we made a video on jade's channel as
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well it's called up and atom i will put
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a link to that in the description check
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it out it's more quantum physics hit
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subscribe if you want more physics
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content right here on physics girl and
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happy physicsing i'm gonna go enjoy a
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concert
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should have come to the concert jade
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[Music]
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you
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